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Ejection of the Envelope
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(Section Not Complete)
HST Gallery of Planetary Nebulae
The adjacent near-infrared
image shows a planetary nebula glowing in the light of hydrogen
that is about 10,000 light years away in the direction of the
constellation Cygnus
(Ref).
Some millions of years ago this star was probably similar to our Sun, but it first evolved into the red
giant phase, in which the center of the star became hotter, driving off the outer layers. The planetary
nebula seen here then resulted when the ejected material was heated by strong UV radiation from the hot
central star until it began to glow. In a few tens of thousands of years, the planetary nebula will be
dispersed into the interstellar medium and the central star will probably cool to become a white dwarf.
The following image shows 6 planetary nebulae photgraphed with the Hubble Space Telescope
(Ref). There appears to be a large
variety of ways in which the central star ejects the outer layers, from spherical symmetry as in IC 3568 to
a pinwheel structure as in NGC 5307.
The image on the left shows a Hubble Space Telescope picture of one of the most beautiful and also
unusual planetary nebula, NGC 6543, which is 3000 light years away in the contellation Draco
(Ref).
It is often called the Cat's Eye Nebula. The complex structure of the
planetary nebula is not well understood. The nebula appears to be about 1000 years old and one
hypothesis is that the central star is actually a binary (though it is not resolved in this image)
and the interaction of the two stars in the binary coupled with strong stellar wind emission has
produced the intricate structure
(Ref).
The adjacent image shows a white dwarf embedded in a planetary nebula that may be the hottest star known.
Its surface appears to have a temperature of about 200,000 degrees C, which is 30 times the surface
temperature of the Sun
(Ref).
Even though the temperature is so high, this is a dying star. It has used all its
nuclear fuel and can no longer produce energy by thermonuclear fusion.
The adjacent image is a false-color radio emission map of the star TT Cygni, which is a cool red giant
star about 1500 light years away in the constellation Cygnus
(Ref). The observed radio emission is from
carbon monoxide (CO) molecules. The large ring, which has a radius of about 1/4 light year, is a shell of
gas exanding outward from the star, which lies at the center. It appears to have been ejected from the
star about 6000 years ago. The central radio emission is from material blown off from the star more
recently (last few hundred years). TT-Cygni is called a carbon star because it has significant
abundances of carbon containing compounds. These were probably produced in the interior by helium burning,
and then brought to the surface by convection.
Such stars lose much of their mass in a strong stellar wind
late in their lives.
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